This application claims the benefit of Korean Patent Application No. 2000-62314, filed on Oct. 23, 2000 in Korea, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method for measuring an image-sticking defect or residual image and for ascertaining whether the image-sticking defect or residual image exists or not.
2. Description of the Related Art
Until now, the cathode-ray tube (CRT) has been generally used for display systems. However, flat panel displays are increasingly beginning to be used because of their small depth dimensions, desirably low weight, and low power consumption requirements. Presently, thin film transistor-liquid crystal displays (TFT-LCDs) are being developed with high resolution and small depth dimensions.
Generally, liquid crystal display (LCD) devices make use of optical anisotropy and polarization properties of liquid crystal molecules to control alignment orientation. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field. Accordingly, when the electric field is applied to liquid crystal molecules, the alignment of the liquid crystal molecules changes. Since refraction of incident light is determined by the alignment of the liquid crystal molecules, display of image data can be controlled by changing the applied electric field.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are of particular interest because of their high resolution and superiority in displaying moving images. Because of their light weight, thin profile, and low power consumption characteristics, LCD devices have wide application in office automation (OA) equipment and video units. A typical liquid crystal display (LCD) panel may include an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, may include a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, may include switching elements, such as thin film transistors (TFTs), and pixel electrodes.
FIG. 1 is a cross-sectional view of a pixel of a conventional LCD panel in an active matrix LCD. As shown, the LCD panel 20 includes upper and lower substrates 5 and 15 and a liquid crystal (LC) layer 10 interposed therebetween. The lower substrate 15 includes a thin film transistor (TFT) xe2x80x9cKxe2x80x9d as a switching element that transmits a voltage to the pixel electrode 14 to change the orientation of the LC molecules. The pixel electrode 14 disposed on a transparent substrate 1 applies an electric field across the LC layer 10 in response to signals applied to the TFT xe2x80x9cK.xe2x80x9d A first alignment layer 6 may be disposed over the TFT xe2x80x9cKxe2x80x9d and pixel electrode 14 adjacent to the LC layer 10. Moreover, the lower substrate 15 may include a storage capacitor 16 that maintains the voltage on the pixel electrode 14 for a period of time.
The upper substrate 5 may include a color filter 2 for producing a specific color and a common electrode 4 disposed over the color filter 2. The common electrode 4 serves as an electrode for producing the electric field across the LC layer (in combination with the pixel electrode 14). The common electrode 4 may be arranged over a pixel portion xe2x80x9cP,xe2x80x9d i.e., a display area. The second alignment layer 7 may be disposed on the common electrode 4. Further, to prevent light leakage of the LC layer 10, a pair of substrates 5 and 15 may be sealed by a sealant 12.
Although FIG. 1 only shows one TFT xe2x80x9cK,xe2x80x9d the lower substrate 15 usually includes a plurality of TFTs as well as a plurality of pixel electrodes each of which electrically contact each of the plurality of TFTs. In the above-described LCD panel 20, the lower substrate 15 and the upper substrate 5 are respectively formed through different manufacturing processes, and then attached to each other. As previously described, the liquid crystal display devices make use of the optical anisotropy and polarization properties of the liquid crystal molecules. Since the liquid crystal molecules are thin and long, and the electric field is applied to the liquid crystal layer, the alignment direction of the liquid crystal molecules can be changed and controlled by the applied electric field. Accordingly, incident light is modulated to display images.
FIG. 2 is a circuit diagram of a conventional active matrix liquid crystal display panel.
In FIG. 2, the active matrix liquid crystal display panel comprises a number of horizontal gate bus lines 32, and a number of vertical data bus lines 42 intersecting the gate bus lines 32, thereby forming a matrix of orthogonal bus lines 32 and 42. One pixel is formed at each intersection of gate and data bus lines 32 and 42. Moreover, a thin film transistor xe2x80x9cKxe2x80x9d is formed at each intersection of the gate and data bus lines 32 and 42 that includes a source electrode xe2x80x9cSxe2x80x9d connected to a corresponding data bus line 42, a gate electrode xe2x80x9cGxe2x80x9d connected to a corresponding gate bus line 32, and a drain electrode xe2x80x9cDxe2x80x9d connected to a storage capacitor xe2x80x9cCstxe2x80x9d and a corresponding individual or pixel electrode of liquid crystal cell xe2x80x9cClc.xe2x80x9d A pixel voltage xe2x80x9cVpxe2x80x9d is applied to the pixel electrode of the liquid crystal cell xe2x80x9cClcxe2x80x9d from the data lines 42 through the TFT xe2x80x9cK.xe2x80x9d A common voltage xe2x80x9cVcomxe2x80x9d is applied to a common electrode that is connected to both the liquid crystal cell xe2x80x9cClcxe2x80x9d and the storage capacitor xe2x80x9cCst.xe2x80x9d In the conventional liquid crystal display panel, the liquid crystal cell xe2x80x9cClcxe2x80x9d and the storage capacitor xe2x80x9cCstxe2x80x9d are connected in parallel. A scanning line driving circuit 30 successively supplies a gate pulse voltage to the gate bus lines 32 with a horizontal scanning period. On the other hand, a signal line driving circuit 40 supplies a pixel signal voltage to the data bus lines 42 in each horizontal scanning period.
The array substrate of the active matrix liquid crystal display panel integrally comprises (mxc3x97n)-number of pixel electrodes 14 (of FIG. 1) arranged in a matrix, an m-number of gate bus lines G1 to Gm arranged along the rows of the pixel electrodes, an n-number of data bus lines D1 to Dn arranged along the columns of the pixel electrodes. Furthermore, an (mxc3x97n)-number of thin film transistors xe2x80x9cKxe2x80x9d are arranged as switching elements in the vicinity of cross points between the gate bus lines G1 to Gm and the data bus lines D1 to Dn corresponding to the (mxc3x97n)-number of the pixel electrodes. The scanning line driving circuit 30 drives these gate bus lines G1 to Gm, and a signal line driving circuit 40 drives the data bus lines D1 to Dn.
Therefore, the scanning line driving circuit 30 successively supplies the gate bus lines 32 with a signal that drives all the gate bus lines G1, G2, . . . Gm to turn on all the TFTs xe2x80x9cKxe2x80x9d arranged in the direction of the column selected by these gate bus lines. The signal line driving circuit 40 also supplies to the data bus lines 42 a signal that drives all the data bus lines D1, D2, . . . Dn to apply a predetermined potential through the data bus lines to all the TFTs xe2x80x9cKxe2x80x9d that have been turned on. When the gate pulse voltage is applied to the gate bus line G1, all the TFTs xe2x80x9cKxe2x80x9d connected to the gate bus line G1 are turned on. At this time, the turned-on TFTs xe2x80x9cKxe2x80x9d electrically connect the data bus lines to the liquid crystal cell xe2x80x9cClcxe2x80x9d and storage capacitor xe2x80x9cCstxe2x80x9d that are electrically connected to the gate bus line G1. As a result, the pixel signal voltage supplied from the signal line driving circuit 40 is applied to the determined liquid crystal cell xe2x80x9cClcxe2x80x9d and storage capacitor xe2x80x9cCst.xe2x80x9d Specifically, the liquid crystal molecules are aligned and oriented by the pixel signal voltage applied to the liquid crystal cell xe2x80x9cClcxe2x80x9d thereby displaying images using the anisotropic characteristics of the liquid crystal molecules.
Thereafter, the gate pulse voltage is applied to the gate bus line G2, thereby turning on the TFTs connected to the gate bus line G2. At this time, the TFTs connected to the gate bus line G1, are turned off. However, the accumulated electricity in the liquid crystal cell xe2x80x9cClcxe2x80x9d and storage capacitor xe2x80x9cCstxe2x80x9d electrically connected to the gate bus line G1 makes the TFTs connected to this gate bus line G1 continue in on-state until the gate pulse voltage is applied to the gate bus line G1 at the next time.
Some problems occur when operating a thin film transistor liquid crystal display using the above-described method. For example, an image-sticking defect may occur when a residual image is displayed as a result of continuously displaying the same image for a long periods of time. The image-sticking defect is commonly caused by a residual direct current (R-DC) voltage generated in the liquid crystal cell xe2x80x9cClcxe2x80x9d as explained in FIGS. 3, 4A and 4B. Furthermore, another cause of the image-sticking defect is reciprocal action of pairs of alignment layers due to electrical stress weakness of the alignment layer.
FIG. 3 is a partial circuit diagram of a conventional pixel of liquid crystal display panel, FIG. 4A is a voltage plot showing the voltages applied to the thin film transistor of the liquid crystal panel, and FIG. 4B is a voltage plot showing the voltage applied to the liquid crystal cell via the thin film transistor. Alignment of liquid crystal molecules deteriorates as a result of application of a direct current voltage. Furthermore, dielectric anisotropy affects the dielectric constant of the liquid crystal cell in accordance with the alignment of the liquid crystal molecules. Accordingly, an alternating current voltage is widely used when driving the thin film transistor.
In FIG. 4A, when employing the above-described method for operating a TFT-LCD, a signal voltage Vd applied to the source electrode xe2x80x9cSxe2x80x9d begins to accumulate in the liquid crystal cell and storage capacitor at the time when the gate pulse voltage Vg is applied to the thin film transistor. Although this accumulated signal voltage Vd should be maintained until a next signal voltage is applied, the accumulated signal voltage Vd is discharged by the parasitic capacitor xe2x80x9cCgsxe2x80x9d (shown in FIG. 3) that is formed between the gate electrode xe2x80x9cGxe2x80x9d and the source electrode xe2x80x9cSxe2x80x9d of the thin film transistor. The discharged voltage xcex94V, shown in FIG. 4B, causes an xe2x80x9coff-setxe2x80x9d direct current voltage to be applied to the liquid crystal cell xe2x80x9cClc.xe2x80x9d Accordingly, the storage capacitor xe2x80x9cCstxe2x80x9d is parallel-connected to the liquid crystal cell xe2x80x9cClcxe2x80x9d to suppress the xe2x80x9coff-setxe2x80x9d direct current voltage. However, the storage capacitor xe2x80x9cCstxe2x80x9d cannot completely control the xe2x80x9coff-setxe2x80x9d direct current voltage, and a portion of the xe2x80x9coff-setxe2x80x9d direct current voltage is applied to the liquid crystal cell xe2x80x9cClc.xe2x80x9d
In FIG. 3, when the direct current voltage is applied to the liquid crystal cell xe2x80x9cClcxe2x80x9d, impurities 52 and 53 are ionized. Positively ionized impurities 52 are adjacent to a negatively polarized alignment layer 51 and negatively ionized impurities 53 are adjacent to a positively polarized alignment layer 54. Over time, the ionized impurities 52 and 53 adhere to the alignment layers. Therefore, the liquid crystal molecules 55 retain their own direct current voltage, i.e., R-DC voltage, due to the ionized impurities 52 and 53 adhering to the alignment layers 51 and 54, respectively. Accordingly, the R-DC voltage in the liquid crystal cell is a major factor causing the image-sticking defect along with the electrical characteristics of the alignment layer. Since the R-DC voltage changes a pretilt angle and alignment of the liquid crystal molecules in the liquid crystal cell, the liquid crystal molecules are not susceptible to the applied signal. Therefore, the image sticking defect occurs when displaying another image after continuously displaying the same image for a long period of time.
The alignment layer is formed of a high molecular compound, such as polyimide, and is disposed adjacent to the liquid crystal layer. The alignment layer is formed by a rubbing process to orient the liquid crystal molecules in one direction. The alignment of the liquid crystal molecules is variable in accordance with the alignment layer. Furthermore, the response of the liquid crystal molecules to the applied electric field is variable in accordance with the alignment layer. Since the alignment layer is electrically susceptible to rubbing conditions, the alignment layer can trap electrical charges. Accordingly, any trapped electrical charges may decrease control of the alignment of the liquid crystal molecules, thereby contributing to the image-sticking defect.
Two causes for the formation of the image-sticking defect, the R-DC voltage, and the electrical characteristics of the alignment layer, may not be readily recognizable. Namely, the two above-described causes for creating the image-sticking defect are related to each other. Furthermore, other factors may cause the image-sticking defect in the TFT-LCD since the LCD device includes many other elements and the LCD device may be fabricated by different processes.
One method for measuring the image-sticking defect includes observation by the naked eye. However, the naked eye observation has an observational error of xc2x12%, and thus it is very difficult to confirm whether or not the image-sticking defect exists. Additionally, observation by the naked eye cannot accurately provide a degree with which the image-sticking defect occurs. Alternatively, there are other methods for measuring the image-sticking defect that use characteristics of the LCD elements. Specifically, the image-sticking defect existence and degree are measured by way of observing the elements of the liquid crystal display that may affect the image-sticking defect. However, among the different methods for measuring the image-sticking defect, the method of measuring R-DC voltage is widely known. The image-sticking defect caused by the electrical characteristics of the alignment layer cannot be effectively measured. Additionally, the method of measuring the variable factors causing the image-sticking defect is not sufficiently developed.
Currently, a method for measuring the R-DC voltage and a voltage holding ratio (VHR) measurement method are known. When a liquid crystal display panel exhibits a R-DC voltage, both the image-sticking defect and flickering occur in the liquid crystal display panel. In order to control and prevent the flickering phenomenon, a voltage opposite in polarity to the xe2x80x9coff-setxe2x80x9d voltage is applied to the liquid crystal cell. In the R-DC voltage measurement method, the xe2x80x9coff-setxe2x80x9d voltage that is applied to the liquid crystal cell by the thin film transistor is measured. According to the voltage holding ratio (VHR) measurement method, a discharged direct current voltage is measured. A voltage stored in the liquid crystal cell is discharged by the resistance of the liquid crystal layer when the TFT is turned on, thereby causing the R-DC voltage. Then, the alternating current voltage applied to the liquid crystal cell and the charged voltage remaining at the liquid crystal cell are measured. From the result of these measurements and the voltage holding ratio, the discharged direct current voltage is theoretically calculated.
In FIGS. 5 and 6, the R-DC voltage measurement method and the VHR measurement method are compared to each other. FIG. 5 is a graph showing relative maximum values of a R-DC voltage according to the R-DC voltage measurement method, and FIG. 6 is a graph showing relative maximum values of a R-DC voltage according to the VHR measurement method. In these graphs, roman numeral I represents a polyimide alignment layer, and roman numeral II to VI represent alignment layers respectively fabricated by different fabrication processes. In order to measure the R-DC voltage, the direct current voltage is successively applied to the liquid crystal cells having the different kinds of alignment layers in a direction from negative to positive (L.R-DC), and then applied in a direction from positive to negative (T.R-DC).
The R-DC voltage and VHR measurement methods are widely used in measuring the image-sticking defect. However, these measurement methods do not consider any intrinsic characteristics of LCD elements. Therefore, although the liquid crystal cells have the same alignment layer when performing the above-described measurement methods, the results are different depending on each of the measurement cases.
Accordingly, the above-described methods using the R-DC voltage is not an adequate measurement method when testing for the existence and degree of the image-sticking defect. Specifically, the existence of the image-sticking defect cannot be clearly known, and the image-sticking defect degree can-not be accurately measured.
Accordingly, the present invention is directed to a method for measuring an image-sticking defect in a liquid crystal display panel that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for measuring an image-sticking defect in a liquid crystal display device.
Another object of the present invention is to provide a method that can measure and quantify an image-sticking in a liquid crystal display device.
Additional features and advantages of the invention will be set forth in the description that follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for measuring an image sticking defect in a liquid crystal display device includes the steps of grounding a liquid crystal cell, the liquid crystal cell including an alignment layer, applying a first alternating current voltage to the liquid crystal cell, measuring a first capacitance of the liquid crystal cell, applying an electrical stress to the liquid crystal cell, measuring a second capacitance of the liquid crystal cell, and calculating a capacitance difference between the first capacitance and the second capacitance.
Another object of the present invention is to provide a method for measuring an image sticking defect in a liquid crystal display device including the steps of applying a first alternating current voltage to a liquid crystal cell, measuring a first capacitance of the liquid crystal cell, electrically connecting a thin film transistor to the liquid crystal cell, applying a second alternating current voltage to the thin film transistor, the second alternating current voltage is selected from a group comprising alternating current voltages each having different waveforms and alternating current voltages each having different voltage values, combining the second alternating current voltage with an off-set voltage occurring in the thin film transistor to obtain a combined voltage, applying the combined voltage to the liquid crystal cell, measuring a second capacitance of the liquid crystal cell, calculating a capacitance difference between the first capacitance and the second capacitance, and calculating a quantified value of the image sticking defect by the following equation:
y=Avr(xcex94C/C1)xe2x88x92xcex1(xcex94C/C1) 
wherein xe2x80x9cyxe2x80x9d is the quantified value of the image sticking defect, C1 is the first capacitance, xcex94C is the capacitance difference between the fist capacitance and the second capacitance, xe2x80x9cAvr(xcex94C/C1)xe2x80x9d is an average of capacitance differences when the second alternating current voltage is applied to the thin film transistor, and xe2x80x9cxcex1xe2x80x9d is a value of the second alternating current voltage applied to the liquid crystal cell.
Another object of the present invention is to provide a method for measuring an image sticking defect in a liquid crystal display device including the steps of applying a first alternating current voltage to a liquid crystal cell, measuring a first capacitance of the liquid crystal cell, electrically connecting a thin film transistor to the liquid crystal cell, applying a second alternating current voltage to the thin film transistor, the second alternating current voltage is selected from a group comprising alternating current voltages each having different waveforms and alternating current voltages each having different voltage values, combining the second alternating current voltage with a direct current voltage having a same value as a residual direct current voltage occurring in the liquid crystal cell to obtain a combined voltage, applying the combined voltage to the liquid crystal cell, measuring a second capacitance of the liquid crystal cell, calculating a capacitance difference between the first capacitance and the second capacitance, and calculating a quantified value of the image sticking defect by the following equation:
y=Avr(xcex94C/C1)xe2x88x92xcex1(xcex94C/C1) 
wherein xe2x80x9cyxe2x80x9d is the quantified value of the image sticking defect, C, is the first capacitance, xcex94C is the capacitance difference between the fist capacitance and the second capacitance, xe2x80x9cAvr(xcex94C/C1)xe2x80x9d is an average of capacitance differences when the second alternating current voltage is applied to the thin film transistor, and xe2x80x9cxcex1xe2x80x9d is a value of the second alternating current voltage applied to the liquid crystal cell.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.